Unknown. First printed usage from 1937. Several unsupported theories exist about its origin. A common explanation, involving Arthur Balfour gaining a promotion through the supposed intercession of his uncle, Robert Cecil, is doubted because the expression did not appear in print until 1930.

Click the Import Contacts button and—Bob's your uncle (that's "Tada!" for my readers)—you should see a confirmation that all went according to plan, and your contacts have been imported into your Gmail address book.